Photolithography is a well known technique in the prior art for selectively etching the surface of a member, the surface of the member comprising a metal or semiconductor material to form a desired pattern on the underlying substrate of the member. Typically, a photoresist is applied to the surface of the material, the photoresist is exposed to light in the desired pattern to harden the photoresist in the areas upon which light is incident (negative photoresist), the member is placed in an etchant to remove the non-hardened areas of the photoresist and the underlying material and the member is then rinsed and the remaining photoresist removed.
The application of photolithographic techniques for forming defined patterns in noble metals such as gold has generally been ignored in the prior art since these metals are difficult to etch using commercially available strong and toxic etchants. Further, although certain traditional etches, such as aqua regia, can etch a gold coating formed on a substrate, for example, this etchant must be applied selectively and directly to the gold coating without contacting other portions of the coating or the substrate since aqua regia is a solvent for most materials. This also precludes the use of many photoresists since aqua regia would remove both the hardened and non-hardened portions of the photoresist.
Other traditional etchants for gold, besides aqua regia, are iodine solutions and alkali cyanide solutions. Aqua regia and elemented iodine are very caustic materials and can potentially produce very toxic chemicals. Accidential acidity of cyanide solutions can produce very posionous hydrogen cyanide gases.
For reasons similar to that described above, photolithography has not been utilized to define conductive patterns in gold coatings when the gold coating is interposed between two layers of material, such as a transparent dielectric substrate and a thin ceramic layer, forming an electrically conductive film.
A film of this type, known as Intrex film, is commercially available from the Sierracin Corporation, Sylmar, California. This film comprises a polyester base on which is deposited a transparent gold coating followed by deposition of a very thin ceramic layer to improve adhesion, scratch resistance and transparency of the film. Films thus formulated are very rugged and have low cost. Major uses for these films include de-icing layers for aircraft windshields and other large area applications. Additional applications for this film can be determined since the film is available in a variety of sheet resistivities which range from approximately 5 to 30 ohms per square, the cost per square foot being generally low.
While many applications for this Intrex film can incorporate cut sheets of the material without any operations on the underlying gold coating as set forth hereinabove, additional applications are only possible if the conductive gold coating is further defined into a pattern which provides a desired device function. For example, touch sensitive keyboards can be made with Intrex film by defining an appropriate and accurate pattern into the gold coating. Similarly, consumable credit cards can be made at low cost using this film by defining a specific and accurate pattern for the required heater array in the gold coating.
The Sierracin Intrex film set forth hereinabove has provided pattern definition on the gold conductive coating by utilizing techniques other than photolithography which include razor blade scribing of the gold coating or electrical burn-off using a pointed stylus coupled to an electrical source. Both of these techniques are tedious, expensive and unreliable and makes the latter recited applications non-cost effective when compared to other conductive films commercially available. Further, the use of aqua regia to etch the gold is not practical since the underlying substrate and/or ceramic layer may be damaged and its use must be handled with extreme care due to its toxic and caustic nature.
It would therefore be desirable if a technique for forming pattern definitions on a thin conductive metal interposed between two layers of material can be provided, and, in particular, to provide a technique for defining a pattern in the Intrex gold coating which is less expensive and less dangerous and yet more reliable and safe than the techniques presently being utilized.